A detailed analysis of the molecular events in meiotic recombination of yeast will be carried out. These studies are based on the physical monitoring by Southern blot analysis of DNa from cells undergoing recombination and on the use of in vitro- generated restriction site mutations introduced into a well-defined chromosomal region. (1) A study of the relation between gene conversion and crossing-over in a well-defined 9 kilobase interval will include a measurement of the length of heteroduplex DNA in strains carrying a mutation that reduces mismatch repair. In addition, a secondary mismatch repair-induced recombination system that we have discovered will be investigated. Deletion analysis of the interval will be conducted to determine if there is a single "hotspot" that initiates the very high level of recombination. Further experiments will also be carried out on a 44 bp region that appears to contain a sequence that acts as a barrier to the extension of (polar) gene conversion events into the region beyond the barrier. (2) Intermediates of meiotic recombination will be identified by physical monitoring techniques. The formation and repair of mismatches in heteroduplex DNA will be examined. A similar approach will be used to investigate the formation and resolution of branched DNa structures (Holliday junctions). (3) We will pursue our study of ectopic recombination between homologous sequences at distant sites. We will extend our studies in Saccharomyces cerevisiae and will carry out similar studies in Schizosaccharomyces pombe. Finally, we will pursue our observation that ectopic recombination between sites on two different chromosomes is sometimes accompanied by chromosomal non- disjunction.